Hi Rod.
I repaired the shower tray on my Abbey Freestyle 540se last year. The van was only 4 years old, but we had been living in it for a year which probably brought about it's demis.
I decided to repair the tray as we had to still use the loo while the works went on, but removal of the whole tray is simlar to what I did. First of all I took out the shower stand and loo roll holders. You wouls have to take apart the swivel loo and its base to get at the whole tray. Using a stanley knife, cut the sealant mid way through the bead width which will put any score lines on the side panels well below the new sealant when applied.
Make a sketch of where all the screws etc go on a note pad as you remove them and keep them safe.
What I found whith my shower tray was that due to compression of the insulation board used to fill the gap under the tray, not supporting all of the tray, it was the most likely cause of the tray failing.
I used two glass fibre kits (Halfords) to re-enforce the underside of the tray all over the foot well area.
To compensate for the poor kingspan board they use under the tray, which is insulation board and not suited to being stood on. I filled all the irregularity's with bathroom sealant. This can be the cheaper houshold type as it is being used more as filler and glue than sealing.
On replacing the shower tray I again put down a bed of sealant on the floor of the van to fill any gaps under the tray.
On finishing the fit of the tray, I used EVERBUILD silicone sealant to do the final seal. This product has a 30 year life span expectancy. We use it in sealing poultry buildings and it is very good quality. If you want a bead radius to look seamless, use an old sealant tube and squeeze the plunger end to form a scoop. Running this along the wet sealant bead makes a very neat finish. To finish any inperfections with your finger wet it first, this avoids icing peaks when you lift your finger off the sealant.
Everbuild have a web site where you can order the sealant.
BTW, my tray is still leak free (fingers X'd) one year on.
Atb Steve L.